The British tourists who reportedly has a grenade thrown at them in Kenya were travelling in a van from Diani in Kwale to Mombasa when it was targeted as it slowed down, police told the Kenya-based Standard Digital.
"He vanished soon after he hurled the explosive," police chief Robert Kitur told the paper.
The grenade failed to explode and is being examined by police.
The attack comes on the same day Kenya celebrates 50 years of independence from British colonialist rule.
More top news
Cool in the north, warmer air to the south
Police have warned the drug, known locally as "pink champagne", could be deadly.
The author said "It's been wonderful" as fans mark two decades since the children's books opened up a world of magic to millions.